Cobalt-base alloy resistant to thermal shock



April 18, 1967 R, MCQUILLAN ETAL 3,314,784

COBALT-BASE ALLOY RESISTANT TO THERMAL SHOCK Filed Nov. 21, 1963 3 Sheets-Sheet l THERMAL SHOCK DISTORTION F COBALT-BASE ALLOY$ CONTAINING CUW, and Ta.

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/ ALLOYS 3,4,5,'H,12,13,2l

CYCLES INVENTORS ROBERT D.MC QUILLAN WILLIAM M. THCMAS ELMER L.FREY [SH/i flfi firhaflgg April 18, 1967 R. D. MQQUILLAN ETAL 3,314,784

COBALT-BASE ALLOY RESISTANT To THERMAL SHOCK Filed Nov. 21, 1963 3 Sheets-5heet 2 INVENTORS ROBERT D.MCQUILLAN WILLIAM M. THOMAS ELMER L.FREY

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A TTOR/VE) April 18, 1967 D MCQUILLAN ETAL 3,314,784

NT TO THERMAL SHOCK COBALT-BASE ALLOY RES IS TA 3 Sheets-$lheet 5 Filed Nov. 21, 1963 IN VENTORS D. McQUILLAN ROBERT WILLIAM M.THOMAS ELMER L.FREY

ATTORNEY United States Patent 3,314,784 COBALT-BASE ALLOY RESISTANT T0 THERMAL SHOCK Robert D. McQuillan, William M. Thomas, and Elmer L. Frey, Kokomo, Ind., assignors to Union Carbide Corporation, a corporation of New York Filed Nov. 21, 1963, Ser. No. 325,338 11 Claims. (Cl. 75-171) This invention relates to cobalt-base alloys for high temperature service and, more particularly, to alloys resistant to extreme conditions of high stresses and thermal shock.

Critical components of certain industrial equipment, such as machines or engines are subject to extreme conditions of heat, stresses, wear, erosion and the like. The so-called super-alloys have been developed to Withstand these forces and cobalt-base alloys are particularly use ful for this purpose. However, it is recognized that the usefulness of any component is ultimately determined by the capacity to Withstand the total combined forces in the actual environment of operation.

One example of this situation exists with regard to alloys used in the vane section of a gas turbine engine. A particular prior art alloy hereinafter referred to as Alloy PA-I has been rather widely used in the manufacture of vanes of gas turbine engines. The nominal composition of PA-I alloy is 21 percent chromium, 10 percent tungsten, 0.9 percent carbon, 9.0 percent tantalum, 0.25 percent zirconium, up to 0.2 percent boron, and the balance cobalt, plus about 0.20 percent silicon There are other cobalt-base alloys known in the art that provide an object of this invention to provide up to about 2100 F., having the comof high strength and thermal shock regraph illustrating the superior thermal shook resistance of alloys of the present invention.

In FIGURE 2, a photomicrograph (original magnification 200X) illustrating the microstructure of preferred alloys of the present invention.

In FIGURE 3, a photomicrograph (original magnification 200X) showing the microstructure of alloy PA-I.

An alloy in a composition accordance with the present invention has of 20 to 35 percent chromium, 5 to 20 I tanium should be kept below 3,314,784 Patented Apr. 18, 1967 percent tungsten, 3 to 5.5 percent tantalum, 0.7 to 1.3 percent carbon, balance cobalt and incidental impurities, the total chromium plus tungsten content of said and 12 times the tantalum content, and the tantalum being present in an amount less than any of the other metal alloying constituents.

Chromium, tungsten and tantalum are included in the allow of the present invention to provide the strength, corrosion and oxidation resistance, hardness and other properties necessary to resist high stresses at temperatures up to about 2100 F. and higher. Carbon must be present in the alloy to form the desired carbides. order to provide the necessary control of the desired carbides, the carbon content must be restricted from about 0.70 to 1.3 percent and preferably 'within the range of 0.75 to 1.2 percent.

It has been discovered as part of the present invention that an extremely critical relationship must exist among the alloying metal elements chromium, tungsten and tantalum. The total chromium and tungsten content Chromium 22.5 to 32.5 percent. Tungsten 8 to 18 percent. Carbon .75 to 1.2 percent. Tantalum 3 to 4.5 percent.

Ti, Zr, B Nil.

Other impurities LoW as possible and less than 5.5%. Balance.

Cobalt compositional range yields alloys that have an excellent combination of properties such as tensile and stress-rupture strength, ductility, oxidat on resistance, thermal shock resistance and good castability characteristics.

The group of elements nickel, molybdenum, iron, silicon, colum bium, manganese, and vanadium are at times adventitiously present in the raw materials used in' preparing this class of cobalt base alloys. These elements, in small quantities, are not particularly harmful in the alloys of this invention, however, because their presence tends to dilute the effective elements and interfere with the relationship of the desired carbides in the structure, it is important that their content in the final alloy be limited to less than 4 percent nickel, less than 0.5 percent low as possible in the final alloy.

Sulfur, magnesium, ziconium, nitrogen. boron and tiabout 0.15 percent in the aggregate. The maximum individual contents for these elements are 0.02%, 0.01%, 0.01%, 0.05%, 0.0 1% and 0.08% respectively. These foregoing elements in excessive amounts are particularly harmful in the alloy of this invention and reduce the resistance to thermal shock deformation and failure. Their presence can be minimized by various techniques such as (1) the proper choice of raw materials used in preparing the alloy, (2) preventing the inter-alloying of deoxidizers with the alloy, (3) the use of non-contaminated crucibles, molds and other materials that contact the molten alloy, and (4) using appropriate melting and casting processing steps, such as hereinafter described.

In this last respect, one unexpected advantage is realized in the processing of the alloy of this invention. For example, high-temperature alloys, now in use, often re quire a controlled content of one or more elements in the group, titanium, boron, zirconium, aluminum, magnesium and nitrogen. As a means of controlling the amount of these elements, it has been found necessary to melt and cast these alloys under a controlled atmosphere such as either an argon gas cover or in vacuum. Often a combination of both is required.

The alloy of the present invention does not require any of these elements in its composition. In fact, best results are obtained when the alloy is entirely free from these elements or at least contains less than about 0.15% in the aggregate. Therefore, there is no need for the extra processing steps involving atmosphere control. It has also been found that melting and casting in ambient air atmosphere yields the best results in the practice of the present invention since any minor amounts of titanium, zirconium, magnesium, aluminum, and nitrogen are practically or completely eliminated when the molten alloy is exposed to ambient atmosphere. In other words, a controlled atmosphere, i.e., vacuum and/or positive argon pressure, usually required for processing the prior art alloys, not only is not required, but actually may be detrimental in the processing of the alloy of the present invention.

To further illustrate the present invention the various alloys listed in Table I were prepared and tested.

TABLE I.-CO1\ 1POSITIOil\ II:,L%F TESTED COBALT-BASE Weight Percent Alloy N0. Or+W Cr ikW Cr W Ta *Alloy 9 also contains 5.5 Mo, 3.45 Fe, and 2.90 Ni; Alloy 10 also contains 4.86 Mo and 2.69 Zr; Alloy PA-I also contains .25 Zr.

The melting of the alloys 124 was conducted in an inductionfurnace in an air atmosphere. Commercially available magnesium and alumina crucibles were used for melting alloys 1-24 and the melting practice to produce about 30 to 50 pounds of final alloy was the same for each of these alloys. This comprised charging the cobalt, tungsten, chromium and carbon into the induction furnace and melting in an air atmosphere at about 2700 F. The melt was deoxidized with usual Ca Si deoxidizer and the tantalum was added to the melt; after interalloying the tantalum, the melt was again deoxidized with titanium. The temperature was then raised to about 2800 F. and the metal was cast into molds.

The PA-I alloy was melted and cast in a complex melting and casting furnace in a vacuum as is customary and required for this alloy in order to obtain optimum properties and also to control the zirconium content.

All of the alloys were cast in forms which provide specimens for various tests. In most cases, the cast specimens were in the form of turbine engine vane components in order to determine the characteristics of the alloy as regards the castability of a practical product. The alloy specimens were subsequently tested in the as-cast condition, i.e., no extra processing or heat treating steps were used or were necessary to obtain optimum properties. The test results shown in Tables II, III, IV, and V and the graph of FIGURE 1 show that alloys of the present invention (e.g. alloys 3, 4, 5, 11, 12, 13 and 21) provide a maximum resistance to thermal shock deformation and failure, yet the other required strength and endurance characteristics are better than, or at least comparable to, the xnown alloys now used in the art.

Alloy 24 of this invention, while not having as high a resistance to thermal shock deformation as the previously mentioned alloys 3, 4, 5, 11, 12, 13, and 21, nevertheless has a very good combination of mechanical properties and thermal shock resistance as shown in Tables II, III and IV.

ALLOYS IN ORDER OF DE- TO DISTORTION (THERMAL Ratio of Carbide Formers Deformation Microstruc- Alloy No. after 500 ture Type Cycles, mils Cr+W, Ta Carbon, percent 3 10. 94 1. 06 A 8 10. 3 89 A 8 8. 89 .85 A 8 9. 73 81 A 11 11. 5 87 A 11 8. 63 A 11 9. 91 89 A 30 4. 75 1. 04 D 30 4. 06 1. 05 C 37 9. 03 1. 04 B 52 3. 63 1. 04 O 57 9. 26 90 D 57 7. 86 .87 D 57 11. 5 1 18 B 57 8. 63 1 18 B 61 4. 06 81 C 63 8. 63 99 E 79 4. 09 88 C 11. 5 33 D 9. 43 57 D 3. 44 90 F Over 150 4. 69 .50 G Over 150 46 H Description of microstructm'cs by group Group: Microconstituents A Continuous network of M 0 type carbides with little or no TaC present; uniform structure.

B Rich in TaC, M 0 type carbides also present.

C Rich in acicular TaC with M C type carbides present.

D Rich in globular TaC with M c type carbides present.

E Rich in globular TaC with M C type carbides, and M C type carbides present.

F Zirconium nitrides and acicular tantalum carbides present; also massive M C carbide formation.

columnar structure.

TABLE III. STRESSRUPOTOUIEIDATA AT 2000 F. AND

Life, hours Elongation, Reduction of Area,

Percent Percent a a R 7. 0 9. 0 8. 5 l4. 5 22. 0 38. 0 15. 2 20. 0 47. 0 l3. 4 15. 1 20. 0 11. 8 l1. 8 20. 9 1. 9 l1. 0 1%.? 6. 2 41. 0 4 s3. 5 5. 5 5. 1 3O 27. 1 8. 0 5. 6 17. 4 13. 5 17. 6 9. (i 10. 3 15. 9 28. 5 6. 4 10. 2 11. 3 8. 9 l0. 5 5. 3 l8. 7 28. 8 26. 3 12. 6 21. 7 3 5 40. 9 7. 6 7. 9

TABLE IV.AVERAGE ROOM TEMPERATURE TENSILE ALLOYS DATA FO R COBALT-BASE Ultimate Reduction Tensile Elongation, of Area, Strengths, Percent Percent X1,000 psi.

TABLE V.-AVERA GE HIGH TEMPERATURE TENSILE PROPERTIES DATA Ultimate Yield Elonga- Reduction Alloy Tempera- Tensile 2% Ofiset, tin of area, turc, F. Strengths, X1,000p.s.i. Percent Percent 1, 700 64 1, 900 38 2, 000 26 1, 700 75 1, s00 45 2, 000 as 1, 700 50 1, 900 30 2, 000 23 Two alloys prepared by the referred process suggested herein p and in .accordance with the present invention were cast in the form of ingots sound forgings. Compositional and hardness data for these alloys are shown in Table VI.

TABLE VI Composition, Weight Percent Hardness, Rc N a Ta Co As-Cast Forged invention have a characteristic structure as compared to other alloys of this general class. The compositional range for these alloys is about 25-30% chromium, about tantalum, about 0.8

graph (original magnification 200X) of alloy 4 of Table I having a composition of 29.99% chromium, 9.88% tungsten, 0.80% carbon, 4.62% tantalum, balance C0.

Further, by way of example, the microstructure of PA-I alloy is shown in FIGURE 3. This is identified as Type F in Table II. In FIGURE 3, massive M C carbide carbide is shown at 7 is shown at 9.

is shown at 5, acicular tantalum and zirconium nitride Tests results have bon content be fixed within the range 0.70 to 1.3% and preferably 0.75 to 1.2% in order to obtain the desired combination of properties. When carbon contents in alloys are outside of the limits of the present invention, suitable distribution and types of carbides are not formed. For example, Alloys 3, 4, 5, 11, 12 and 13 contain about 0.85% carbon with about 40% (CH-W) and Alloy 21 contains 1.06% carbon With about 45% (Cr+W). Thus, the ratio of carbon to carbide formers remains about the same and these alloys have the desired combination of properties. On the other hand, alloys 1, 2, 6, 7 and others that contain carbon either above or below the suggested limits, while having apparently similar compositions to the aforementioned alloys do not have the same combination of mechanical strength properties.

Also as previously mentioned, the tantalum content is also critical in the alloys of this invention, since with alloys having less than about 3% tantalum, the corrosion resistance is marginal or inadequate. On the other hand, alloys containing more than about 5.5% tantalum contain undesirable amounts of acicular and/ or globular tantalum carbides in their microstructure and consequently do not withstand the effects of thermal shock. By way of example, Alloys 12 and 14 are similar except for the tantalum content. However, alloy 12 with about 4.5% tantalum has excellent properties, whereas Alloy 14 with about 9.7% tantalum has inadequate resistance to thermal shock.

The data of Table II relating to distortion resistance (thermal shock resistance) were obtained by using test apparatus and test procedures well known in the art. Briefly the test used is as follows: Eight airfoil-shaped specimens are mounted on a hub, which rotates at 1725 rpm. and travels between a furnace and a water spray quenching station at preset time intervals. About 400 cc. of aspirated water is used as the quenching agent. Each cycle consists of 60 seconds exposure in the furnace, which operates at 2100 F., and 90 seconds cooling in the water spray. The specimens are heated to 2100 F. in the furnace in about 40 seconds. After each 100 cycles, the test is halted and the specimens are measured for deformation and are visually examined for cracking or other anomalies. Five hundred cycles constitute a complete test. Results of the test are graphically presented in FIGURE 1 and tabulated in Table II. FIGURE 1 shows the average deformation or bow of each specimen versus the number of cycles.

The deformation or bow is determined by the actual measured dimensional deviation from the original shape resulting from the thermal shock conditions of the test. The standard air-foil shaped specimens used in this test (one-inch width, two-inch length) are measured for deviation at 1.85 inches from the hub. FIGURE 1 clearly shows that the specimens made of alloys of this invention, alloys 3, 4, 5, 11, 12, 13, and 21 have much higher resistance to deformation resulting from thermal shock than the other tested alloys.

What is claimed is:

1. A thermal-shock resistant cobalt base alloy consisting essentially of 20 to 35 percent chromium, to 20 percent tungsten, 3 to 5.5 percent tantalum, 0.7 to 1.3 percent carbon, a maximum of 0.08 percent titanium, a maximum of 0.01 percent zirconium, less than 1% columbium, balance cobalt and incidental impurities, the total chromium plus tungsten content of said alloy being greater than about 35 percent and not exceeding about 47.5 percent; the tungsten content being greater than about two times the tantalum content, the total chromium and tungsten content being between 8 and 12 times the tantalum content, and the tantalum being present in an amount less than the tungsten content and less than the chromium content.

2. An alloy in accordance with claim 1 wherein the carbon content is between about 0.75 and about 1.2 percent.

3. An alloy in accordance with claim 1 wherein the total chromium and tungsten content is about times the tantalum content.

4. A thermal-shock resistant cobalt base alloy consisting essentially of about 25 to about percent chromium, about 9.9 to about 15 percent tungsten, about 3.5 to about 4.6 percent tantalum, about 0.8 to about 1.1 percent carbon, a maximum of 0.8 percent titanium, a maximum of 0.01 percent zirconium, less than 1% columbium, balance cobalt and incidental impurities, the total chromium plus tungsten content of said alloy being greater than about percent and not exceeding about percent; the tungsten content being greater than about two times the tantalum content, the total chromium and tungsten content being between 8 and 12 times the tantalum content, and the tantalum being present in an amount less than the tungsten content and less than the chromium content said alloy being characterized by a microstructure having a substantially continuous network of carbides, at least 99% of said carbides being M C type carbides.

5. A thermal-shock resistant cobalt base alloy consisting essentially of about 30 percent chromium, about 10 percent tungsten, about 3.5 percent tantalum, about 0.9 percent carbon, a maximum of 0.08 percent titanium, a maximum of 0.01 percent zirconium, less than 1% columbium, balance cobalt and incidental impurities said alloy being characterized by a microstructure having a substantially continuous network of carbides, at least 99% of said carbides being M C type carbides.

6. A thermal-shock resistant cobalt base alloy consisting essentially of about 30 percent chromium, about 10 percent tungsten, about 4.6 percent tantalum, about 0.8 percent carbon, a maximum of 0.08 percent titanium, a maximum of 0.01 percent zirconium, less than 1% columbium, balance cobalt and incidental impurities said alloy being characterized by a microstructure having a substantially continuous network of carbides, at least 99% of said carbides being M C type carbides.

7. A thermal-shock resistant cobalt base alloy consisting of about 30 percent chromium, about 10 percent tungsten, about 4.0 percent tantalum, about 0.9 percent carbon, a maximum of 0.08 percent titanium, a maximum of 0.01 percent zirconium, less than 1% columbium, balance cobalt and incidental impurities said alloy being characterized by a microstructure having a substantially continuous network of carbides, at least 99% of said carbides being M C type carbides.

8. A thermal-shock resistant cobalt base alloy consisting essentially of about 25 percent chromium, about 15 percent tungsten, about 3.9 percent tantalum, about 0.9 percent carbon, a maximum of 0.08 percent titanium, a maximum of 0.01 percent zirconium, less than 1% columbium, balance cobalt and incidental impurities said alloy being characterized by a microstructure having a substantially continuous network of carbides, at least 99% of said carbides being M C type carbides.

9. A thermal-shock resistant cobalt base alloy consisting essentially of about 25 percent chromium, about 15 percent tungsten, about 4.6 percent tantalum, about 0.9 percent carbon, a maximum of 0.08 percent titanium, a maximum of 0.01 percent zirconium, less than 1% columbium, balance cobalt and incidental impurities said alloy being characterized by a microstructure having a substantially continuous network of carbides, at least 99% of said carbides being M C type carbides.

10. A thermal-shock resistant cobalt base alloy consisting essentially of about 25 percent chromium, about 15 percent tungsten, about 4.1 percent tantalum, about 0.8 percent carbon, a maximum of 0.08 percent titanium, a maximum of 0.01 percent zirconium, less than 1% columbium, balance cobalt and incidental impurities said alloy being characterized by a microstructure having a substantially continuous network of carbides, at least 99% of said carbides being M C type carbides.

11. A thermal-shock resistant cob-alt base alloy consisting essentially of about 30 percent chromium, about 15 percent tungsten, about 4.1 percent tantalum, about 1.1 percent carbon, a maximum of 0.08 percent titanium,

9 a maximum of 0.01 percent zirconium, less than 1% columbium, balance cobalt and incidental impurities said alloy being characterized by a rnicrostructure having a substantially continuous network of carbides, at least 99% of said carbides being M C type carbides.

References Cited by the Examiner UNITED STATES PATENTS 2,974,036 3/1961 Thielemann 75-171 10 3,118,763 1/1964 Thielemann 75-171 FOREIGN PATENTS 830,649 3/1960 Great Britain. 5 838,835 6/1960 Great Britain.

DAVID L. RECK, Primary Examiner. WINSTON A. DOUGLAS, Examiner. C. M. SCHUTZMAN, R. O. DEAN, Assistant Examiners. 

1. A THERMAL-SHOCK RESISTANT COBALT BASE ALLOY CONSISTING ESSENTIALLY OF 20 TO 35 PERCENT CHROMIUM, 5 TO 20 PERCENT TUNGSTEN, 3 TO 5.5 PERCENT TANTALUM, 0.7TO 1.3 PERCENT CARBON, A MAXIMUM OF 0.08 PERCENT TITANIUM, A MAXIMUM OF 0.01 PERCENT ZIRCONIUM, LESS THAN 1% COLUMBIUM, BALL ANCE COBALT AND INCIDENTAL IMPURITIES, THE TOTAL CHROMIUM PLUS TUNGSTEN CONTENT OF SAID ALLOY BEING GREATER THAN ABOUT 35 PERCENT AND NOT EXCEEDING ABOUT 47.5 PERCENT; THE TUNGSTEN CONTENT BEING GREATER THAN ABOUT TWO TIMES THE TANTALUM CONTENT, THE TOTAL CHROMIUM AND CONTENT BEING BETWEEN 8 AND 12 TIMES THE TANTALUM CONTENT, AND THE TANTALUM BEING PRESENT IN AN AMOUNT LESS THAN THE TUNGSTEN CONTENT AND LESS THAN THE CHROMIUM CONTENT. 